Field of Invention
The present invention relates to a system of zoned catalysts and methods for treating combustion exhaust gas.
Description of Related Art
Combustion of hydrocarbon-based fuel in engines produces exhaust gas that contains, in large part, relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2). But the exhaust gases also contain, in relatively small part, noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of flue and exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of the undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
Typically, exhaust gases from lean burn gas engines have a net oxidizing effect due to the high proportion of oxygen that is provided to ensure adequate combustion of the hydrocarbon fuel. In such gases, one of the most burdensome components to remove is NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). The reduction of NOx to N2 is particularly problematic because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. Notwithstanding, NOx can be reduced by a process commonly known as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a reducing agent, such as ammonia, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for stoichiometric SCR reactions using ammonia is:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Zeolites having an exchanged transition metal are known to be useful as SCR catalysts. Conventional small pore zeolites exchanged with copper are particularly useful in achieving high NOx conversion at low temperatures. However, the interaction of NH3 with NO absorbed onto the transition metal of an exchanged zeolite can lead to an undesirable side reaction that produces N2O. This N2O is particularly problematic to remove from the exhaust stream. Accordingly, there remains a need for improved methods that result in a high conversion of NOx with minimal N2O production. The present invention satisfies this need amongst others.